The present invention relates to a signalling system for exchanging data between signalling units by means of data messages, each of said units including message send and receive means and said signalling system including control means cooperating with said signalling units for the transmission of said messages.
Such a signalling system is already known from the articles "AXE-10 System description" by M. Eklund et al, and "AXE-10 Software structure" by G. Hemdal, both published in Ericsson Review, No. 2, 1976, pages 70-89 and 90-99 respectively, this function block oriented stored programme controlled system being also disclosed in U.S. Pat. No. 3,969,701.
This known signalling system, part of this stored programme controlled telecommunication switching system, includes signalling units comprising so-called software function units each with its own data and its own programme having only access to its own data. The modularity thus obtained considerably enhances the reliability of the system because in this way data of a function unit cannot be altered by the programme of another function unit. But then data messages must be exchanged between the various function units, such a message transfer being obviously a potential source of errors.
In the above mentioned system it might be required to substitute a new function unit for another one, e.g. for an erroneously operating function unit. Although in the above mentioned literature no detailed information is given on message transfer, it is clear that in the known system such a modification will give rise to problems because such a modification normally affects the contents of the messages to be transmitted to the new function unit. Indeed, it will then be necessary to adapt all the function units originating these messages to the new situation and this obviously is a complex operation which can therefore not be performed on-line i.e. while the system is operating.